A standard turbocompound engine includes a turbocharged diesel engine with a power turbine placed downstream of the turbine of the turbocharger. The power turbine recovers exhaust energy and transmits power to the engine output shaft, to which the turbine is coupled by a gear train.
While the power turbine is useful in providing additional output shaft power, it has been found that the power turbine restricts air flow to unacceptable levels under certain operating conditions, such as at high altitudes, during low speed/high load conditions and during transient load conditions. The insufficient air flow results in poor performance, high exhaust temperatures and increased smoke or particulate emissions.
Exhaust valving systems which attempt to provide more favorable exhaust pressures for various operating conditions are well known. For example, U.S. Pat. No. 4,391,098 to Koduge discloses a turbocompound engine in which first and second waste gates are provided to control the supercharging pressure and the pressure fed to an auxiliary (power) turbine. The waste gates include pressure sensitive valves which open when the pressure is sufficient to overcome the bias of a spring plus atmospheric pressure. In operation, the first waste gate is opened when the supercharging pressure reaches a predetermined level, and bypasses a portion of the engine exhaust past the turbocharger turbine to the auxiliary turbine. When the engine operates at high speed, high load, the second waste gate is opened bypassing at least a portion of the exhaust gases past the auxiliary turbine. However, such a valving system does not provide improved exhaust conditions for various engine operating conditions, for example, to provide a better turbocharger match at low speed/high load conditions or during transient load conditions. The Kosuge arrangement merely provides relief valves to relieve pressures above a predetermined limit.
Japanese Utility Model Laid Open No. 157,941/85 discloses a turbocompound engine in which a power turbine bypass is actuated in response to depression of the accelerator pedal. However, actuation of a bypass valve based upon accelerator pedal positioning alone is insufficient to provide satisfactory exhaust flow for various operating conditions such as high altitude, low speed/high load or transient load conditions.
U.S. Pat. Nos. 4,748,812 and 4,800,726 to Okada et al. disclose turbocompound engines in which the exhaust leading to the auxiliary turbine is blocked during exhaust braking and the auxiliary turbine is utilized as a compressor driven by the engine crankshaft. In this arrangement, the blockage of the exhaust results in high energy back pressure which acts as an engine brake force by increasing the pumping work of the engine. However, the patent is not concerned with providing improved exhaust system conditions during non-braking modes and Oakda does not provide improved exhaust flow for various operating conditions since it merely blocks the exhaust flow.